Until recently, charge-coupled devices (CCD's) were the only image sensors widely used in digital imaging. They have been well developed through their use in astronomical telescopes, scanners, and video camcorders. However, CMOS image sensors promises to eventually become the image sensor of choice in a large segment of the digital imaging market. Both CCD and CMOS image sensors capture light on a grid of small pixels on their surfaces. It's how they process the image and how they are manufactured where they differ from one another. CCD Image Sensors
CCD's got their name from the way the charges on the pixels are read after an exposure. After the exposure the charges on the first row are transferred to a place on the sensor called the read out register. From there, the signals are fed to an amplifier and then on to an analog-to-digital converter. Once the row has been read, its charges on the readout register row are deleted, the next row enters, and all of the rows above march down one row. The charges on each row are “coupled” to those on the row above so when one moves down, the next moves down to fill its old space. In this way, each row can be read-one row at a time.